In wireless location systems where it is essential to absolutely locate a unit being tracked or located (e.g. wireless tag) in a limited and well defined area, area or room differentiation is traditionally done in one of several different manners. For example, area or room differentiation may be accomplished by measuring the strength of the received signal, by RFID methods, or by detecting infrared or ultrasonic signals.
The “signal strength” (Received Signal Strength Indication—RSSI) method's differentiation is poor in terms of accuracy and reliability, as the path loss is highly dependent on environmental conditions and affected by fading due to multi path propagation. In indoor office environments, where room differentiation is required, the reliability of this method is highly dependent on the wall materials and the level of RF signal attenuation between two adjacent areas (e.g. two sides of a wall). In outdoor and/or open areas (e.g. manufacturing areas, warehouses, etc.), this method is completely unreliable and does not permit to absolutely differentiate between two adjacent areas (e.g. separated by a fence, canal or low wall).
RFID methods use short range transmitters or readers to detect tags in their proximity. Those methods are reliable both in closed or open areas but require a heavy infrastructure to fully cover the located areas.
Other methods using infrared and/or ultrasonic signals are also very unreliable in outdoor and/or open areas. In indoor closed areas they perform much better but require constant coverage of the applicable sensors. Infrared systems require line-of-sight while ultrasonic systems are limited in range and exposed to several interference problems.
Some systems use tracking algorithms to improve area differentiation, but those methods normally require frequent location update. If the update rate of the location is several minutes or even hours a reliable tracking becomes very difficult.
Therefore, it would be desirable to provide a method that overcomes the problems associated with prior art methods. The method will allow for absolute area differentiation without the need of expensive infrastructure deployment. The method will allow dynamic programming of a transmitter (tracked unit) to allow the location system to absolutely differentiate between areas.